Liquid crystal display device

ABSTRACT

Light-emitting elements can be accurately directly opposed to the entrance surface of a light guide at an accurate distance therefrom, and can be mounted on a single side of a printed circuit board together with other electronic components, whereby the number of manufacturing steps and the thickness of a liquid crystal display device can be reduced. The light guide and the printed circuit board are disposed on the back surface of the liquid crystal display panel, and the respective light-emitting elements are inserted through through-holes formed to extend through the printed circuit board, with the light-emitting portions of the respective light-emitting elements opposed to the entrance surface of the light guide. The light-emitting elements, together with the other electronic components, are mounted on the printed circuit board from one side. Electrodes of the light-emitting elements are bridged and secured to a mounting surface of the printed circuit board.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/712,919 filed on Mar. 2, 2007, which is a Continuation of U.S.application Ser. No. 10/964,718 filed on Oct. 15, 2004, which is aContinuation of U.S. application Ser. No. 10/141,140 filed on May 9,2002. Priority is claimed based on U.S. application Ser. No. 11/712,919filed on Mar. 2, 2007, which claims the priority of U.S. applicationSer. No. 10/964,718 filed on Oct. 15, 2004, which claims the priority ofU.S. application Ser. No. 10/141,140 filed on May 9, 2002, which claimspriority to Japanese Patent Application No. 2001-163017, filed on May30, 2001, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display device and,more particularly, to a liquid crystal display device which is providedwith an auxiliary light source device having a light guide withsolid-state light-emitting elements such as light-emitting diodesdisposed at a side edge of the light guide as a light source.

In small-sized information terminals such as mobile telephones or mobileinformation terminals, it is general practice to adopt, as their displaydevices, liquid crystal display devices which are energy efficient,small-sized and light weight. The liquid crystal display devices areclassified into two types: one type in which external light is utilizedas illuminating means for visualizing an electronic latent image formedon a liquid crystal display panel; and the other type in which anauxiliary illuminating device is disposed on the back or front surfaceof a liquid crystal display panel. The auxiliary illuminating devicedisposed on the back surface of the liquid crystal display panel iscalled a backlight, while the auxiliary illuminating device disposed onthe front surface of the liquid crystal display panel is called afrontlight.

As a light source for this kind of auxiliary illuminating device of sucha mobile terminal, there is a light source which has a light guide witha cold cathode fluorescent tube disposed at a side edge of the lightguide, as in a notebook personal computer having a comparatively largedisplay screen size. However, many mobile telephones and small-sizedmobile information terminals (such as PDAs) use solid-statelight-emitting elements such as light-emitting diodes (LEDs) with lowpower consumption.

FIG. 11 is a diagrammatic explanatory perspective view of the entireconstruction of a related-art auxiliary illuminating device in asmall-sized information terminal. This auxiliary illuminating device isdisposed on the back surface of a liquid crystal display panel. Theliquid crystal display panel, a printed circuit board and the like arenot shown. Two prism sheets 2 are disposed directly below the liquidcrystal display panel, and a diffusing sheet 3, a light guide 4 and areflecting sheet 10 are disposed in that order below the two prismsheets 2. Reference numeral 8 denotes a light-emitting element whichemits white light, and in general, a light-emitting diode (LED) is used.In the following description, this kind of light-emitting element isreferred to as an LED.

FIGS. 12A and 12B are explanatory views of a structural example in whichLEDs constituting the light source of the related-art auxiliaryilluminating device are disposed in a small-sized information terminal.FIG. 12A is a plan view, and FIG. 12B is a cross-sectional view takenalong line A-A of FIG. 12A. In FIGS. 12A and 12B, reference numeral 4denotes a light guide, reference numeral 4A denotes a side edge(hereinafter referred to also as an entrance surface). Reference numeral6 denotes a box-like case (molded case), and reference numeral 8 denotesan LED. In this structural example, one or two or more LEDs 8 aredisposed on the inside wall of the molded case 6. In this example, threeLEDs 8 are arranged.

However, in this construction, since the light guide 4 and the LEDs 8are incorporated in the molded case 6 as separate components, it isdifficult to mount the LEDs 8 accurately on the inside wall of themolded case 6, and a variation easily occurs in a distance d between thelight-emitting portions of the respective LEDs 8 (that lie on thesurfaces of the respective LEDs 8 opposed to the entrance surface 4A ofthe light guide 4) and the entrance surface 4A of the light guide 4. Inaddition, the light-emitting parts and the entrance surface 4A of thelight guide 4 are difficult to oppose precisely directly. For thisreason, the distribution of light which is emitted from the LEDs 8 andenters the light guide 4 becomes nonuniform therein, so thatirregularity may occur in the display luminance of the liquid crystaldisplay panel. This has been one problem to be solved in the relatedart.

Moreover, because of the structure in which the LEDs 8 are mounted inthe molded case 6, wiring means is needed for connecting power supplylines for the LEDs 8 to power source terminals (or power sourceelectrodes) of driver circuits mounted on a printed circuit board (notshown), so that the assembling process becomes complicated. This hasbeen another problem to be solved in the related art. Incidentally, thiskind of related art is disclosed in, for example, Japanese PatentLaid-Open No. 133426/1999.

To solve the above-described problem, various proposals which will bedescribed below have been made. FIG. 13 is a diagrammatic explanatorycross-sectional view of an example of the essential construction of aproposed example of an auxiliary illuminating device in a small-sizedinformation terminal. FIG. 13 mainly shows the light guide 4 of theauxiliary illuminating device and the LEDs 8 which are light-emittingelements. In this construction, the LEDs 8 to be disposed in proximityto or in intimate contact with one end (the side edge 4A) of the lightguide 4 suitably made of a transparent resin are mounted on a printedcircuit board 5, and this printed circuit board 5 is disposed on theback surface of the light guide 4. Incidentally, as the printed circuitboard 5, a non-flexible type made of a hard plate or a type made of aflexible sheet (a so-called flexible printed circuit board) isavailable, and in the following description, both types will becollectively referred to as “printed circuit board”.

Since the LEDs 8 are ordinarily mounted on the printed circuit board 5by an automatic mounting apparatus, their mounting positions can becontrolled accurately. In addition, the light guide 4 is secured to theback surface of a liquid crystal display panel (not shown) by anadhesive agent or a double-sided adhesive tape, and the printed circuitboard 5 is constructed to be connected to the liquid crystal displaypanel. Accordingly, light-emitting portions 8A of the respective LEDs 8can be disposed to be accurately directly opposed to the side edge 4Awhich is the entrance surface of the light guide 4. Incidentally,although not shown, a reflection-processed film or a reflecting sheetare disposed on the back surface of the light guide 4.

FIG. 14 is a diagrammatic explanatory view of an example of theessential construction of a liquid crystal display device, aiding inexplaining the problem of the construction shown in FIG. 13. FIG. 14 isan explanatory cross-sectional view particularly showing the arrangementrelationship between the printed circuit board 5 on which the LEDs 8 amounted as the light-emitting elements of the auxiliary illuminatingdevice, and the light guide 4 as well as a liquid crystal display panel1. The LEDs 8 are disposed in proximity to or in intimate contact withthe side edge 4A of the light guide 4 disposed on the back surface ofthe liquid crystal display panel 1. Although the printed circuit board 5is herein described as a hard printed circuit board, the samedescription applies to a flexible printed circuit board.

The LEDs 8 are mounted on the printed circuit board 5 with thelight-emitting portions 8A facing the side edge 4A of the light guide 4.Light emitted from the light-emitting portions 8A of the LEDs 8 isdirected in a direction approximately parallel to the plane of theprinted circuit board 5. The respective LEDs 8 have electrodes 8B attheir bottoms, and these electrodes 8B are connected to the wiring ofthe printed circuit board 5, whereby the LEDs 8 are secured andelectrically connected to the printed circuit board 5. Other electroniccomponents 9 are mounted on the surface of the printed circuit board 5opposite to the surface on which the LEDs 8 are mounted. In other words,the LEDs 8 and those other electronic components 9 are mounted on theopposite sides of the printed circuit board 5, and this mounting methodis called dual-sided mounting.

FIG. 15 is a diagrammatic explanatory view of an example of theessential construction of another proposed example of the auxiliaryilluminating device in the small-sized information terminal. FIG. 15 isan explanatory cross-sectional view particularly showing the arrangementrelationship between the printed circuit board 5 on which the LEDs 8 aremounted and the light guide 4 as well as the liquid crystal displaypanel 1. The LEDs 8 are mounted on the same surface of the printedcircuit board 5 as the other electronic components 9 and at a portionnear one end of the printed circuit board 5. The portion where the LEDs8 are mounted is folded in the direction of the liquid crystal displaypanel 1, and in a manner similar to that shown in FIG. 14, therespective light-emitting portions 8A of the LEDs 8 are disposed inproximity to or in intimate contact with the side edge 4A of the lightguide 4 disposed on the back surface of the liquid crystal display panel1, in such a manner that the light-emitting portions 8A are opposed tothe side edge 4A.

The LEDs 8 are mounted on the printed circuit board 5 with thelight-emitting portions 8A facing the side edge (entrance surface) 4A ofthe light guide 4. Light emitted from the light-emitting portions 8A isdirected in a direction approximately parallel to the plane of theprinted circuit board 5 and enters the entrance surface 4A of the lightguide 4. The respective LEDs 8 have the electrodes 8B at their bottoms,and these electrodes 8B are connected to the wiring of the printedcircuit board 5, whereby the LEDs 8 are secured and electricallyconnected to the printed circuit board 5. The other electroniccomponents 9 are mounted on the same surface as the surface where theLEDs 8 are mounted. In this example, the LEDs 8 and the other electroniccomponents 9 are mounted on the same surface of the printed circuitboard 5, and this mounting method is called single-sided mounting.

Incidentally, a related art associated with the mounting of the LEDs 8on the printed circuit board 5 is disposed in Japanese Patent Laid-OpenNo. 77724/2000. This laid-open document does not have any disclosure asto liquid crystal display devices. The mounting art disclosed in thislaid-open document relates to the mounting of a single light-emittingelement or of a light-emitting element and a light-receiving element,and aims at reducing the total thickness of a printed circuit board.However, the light-emitting direction of the light-emitting element is adirection perpendicular to the plane of the printed circuit board, andis not intended to solve problems to be solved by the invention, whichoccur when emitted light is made to enter a light guide in a directionparallel to a printed circuit board.

The example of mounting the LEDs in the auxiliary illuminating deviceaccording to each of the above-described proposals has the followingproblems. For instance, in the case of the dual-sided mounting in whichthe LEDs 8 are mounted on the surface of the printed circuit board 5opposite to the surface where the other electronic components 9 aremounted, as shown in FIG. 14, mounting work for the printed circuitboard 5 is complicated, and the process of performing mounting on twosurfaces is needed in the automatic mounting apparatus. In the casewhere the single-sided mounting type of printed circuit board 5described above with reference to FIG. 15 is used, the portion where theLEDs 8 are mounted needs to be folded so that the light-emittingportions 8A are opposed to the side edge 4A of the light guide 4.Accordingly, a folding process is needed, so that work efficiency lowersand cracks or the like may occur in a folded portion, incurringmalfunction. In addition, the thickness of the auxiliary illuminatingdevice increases, with the result that this increase of the thicknesshinders a further reduction in the entire thickness of a liquid crystaldisplay device in which such an auxiliary illuminating device isincorporated, and hence a further reduction in the thickness of aterminal using such a liquid crystal display device.

Moreover, in each of the above-described mounting methods, since theLEDs 8 are mounted on the mounting surface of the printed circuit board5, the distance between the light-emitting portions 8A of the respectiveLEDs 8 and the surface of the printed circuit board 5 can only bereduced within a limited range beyond which no further reduction inthickness is possible. Since the respective light-emitting portions 8Aof the LEDs 8 lie inside the molded surfaces of the LEDs 8, thethickness of the light guide 4 can only be made smaller than the size ofthe light-emitting portion 8A of each of the LEDs 8 directly opposed tothe light guide 4, within a limited range in terms of the efficiency ofutilization of illuminating light. These limitations are also a causewhich hinders a further reduction in the thickness of the auxiliaryilluminating device as well as a further reduction in the thickness ofthe entire terminal in which the auxiliary illuminating device isincorporated.

The invention solves the problems of the above-described related artsand provides a liquid crystal display device in which light-emittingelements such as LEDs which constitute an auxiliary illuminating devicecan be mounted on a printed circuit board together with other electroniccomponents by single-sided mounting and the light-emitting elements canbe opposed to the entrance surface of a light guide without the need tofold the printed circuit board, whereby further reductions in the numberof manufacturing steps and in the thickness of the liquid crystaldisplay device can be promoted.

SUMMARY OF THE INVENTION

Therefore, a basic concept of the invention resides in a construction inwhich a printed circuit board on which to mount a light-emitting elementis provided with a through-hole through which to insert thelight-emitting element and the light-emitting element is mounted in thethrough-hole by single-sided mounting. The light-emitting element andthe other electronic components are mounted on the same surface of theprinted circuit board. The typical constructions of the invention are asfollows.

(1) A liquid crystal display device includes a liquid crystal displaypanel, a printed circuit board on which a plurality of electroniccomponents including a light-emitting element are mounted, and a lightguide provided between the liquid crystal display panel and the printedcircuit board. The printed circuit board has a through-hole, and thelight-emitting element is mounted on the printed circuit board from thesame side as other electronic components with a light-emitting portionof the light-emitting element projected from the through-hole into aside opposite to the other electronic components.

(2) In (1), the light-emitting portion of the light-emitting element isprojected from the printed circuit board into a side where the lightguide is disposed.

(3) In (1) or (2), a light-emitting direction of the light-emittingelement is approximately parallel to a surface of the printed circuitboard.

(4) In any of (1) to (3), the printed circuit board is a flexibleprinted circuit board.

According to the construction described in (1) to (4), on the printedcircuit board on which the light-emitting element is mounted from thesame side together with the other electronic components, thelight-emitting portion of the light-emitting element can be directlyopposed to the entrance surface of the light guide without the need tofold the printed circuit board in the portion of the light-emittingelement, whereby the number of manufacturing steps can be reduced. Inaddition, since a flexible printed circuit board is used, it is possibleto easily provide electrical connection between the printed circuitboard and the liquid crystal display panel.

(5) A liquid crystal display device includes a liquid crystal displaypanel, a printed circuit board on which a light-emitting element ismounted, and a light guide provided between the liquid crystal displaypanel and the printed circuit board. The printed circuit board has athrough-hole, and the light-emitting element which has a light-emittingdirection approximately parallel to a surface of the printed circuitboard is mounted in the state of being inserted through thethrough-hole.

(6) In (5), an end of the light-emitting portion of the light-emittingelement on the side of the printed circuit board is approximately equalin position to the surface of the printed circuit board.

(7) In (5) or (6), the light guide is smaller in thickness than thelight-emitting element in portion thereof which is respectivelyprojected from the through-hole of the printed circuit board.

(8) In any of (5) to (7), the printed circuit board is a flexibleprinted circuit board.

According to the construction described in (5) to (8), the thickness ofa liquid crystal display device can be reduced.

(9) A liquid crystal display device includes a liquid crystal displaypanel, a printed circuit board on which a plurality of electroniccomponents including a light-emitting element are mounted, and a lightguide provided between the liquid crystal display panel and the printedcircuit board. The printed circuit board has a through-hole throughwhich the light-emitting element is inserted, and electrodes of thelight-emitting element and electrodes of other electronic components areconnected to the printed circuit board on a side of the printed circuitboard opposite to the light guide.

(10) In (9), the printed circuit board is a flexible printed circuitboard.

According to the construction described in (9) or (10), thelight-emitting portion of the light-emitting element can be accuratelydirectly opposed to the entrance surface of the light guide without theneed to fold the printed circuit board on which the light-emittingelement is mounted, and the light-emitting element and the otherelectronic components can be connected to the printed circuit board inthe same process, whereby the number of manufacturing steps can bereduced. In addition, since a flexible printed circuit board is used, itis possible to easily provide electrical connection between the printedcircuit board and the liquid crystal display panel.

(11) A liquid crystal display device includes a liquid crystal displaypanel, a printed circuit board on which a plurality of light-emittingelements are mounted, and a light guide provided between the liquidcrystal display panel and the printed circuit board. The printed circuitboard has a plurality of through-holes approximately equidistant from anend of the light guide, and the plurality of light-emitting elements aremounted in the state of being inserted through the respectivethrough-holes.

(12) In (11), the through-holes and the light-emitting elements havepositioning means.

(13) In (11) or (12), the printed circuit board is a flexible printedcircuit board.

According to the construction described in (11) to (13), by bringing theplurality of light-emitting elements into abutment with positioningshapes specially provided on the inner peripheries of the respectivethrough-holes or the respective through-holes, the positions of thelight-emitting elements relative to the entrance surface of the lightguide are restricted, whereby the distance between the light-emittingelements and the entrance surface of the light guide can be maintainedat a design value and the occurrence of luminance irregularity on thedisplay screen of the liquid crystal display panel can be restrained.

Incidentally, the liquid crystal display panel used in the invention isnot limited to a so-called simple matrix type, and may also use anactive matrix type of liquid crystal display panel such as a thin filmtransistor type or various other types of liquid crystal display panels.

Moreover, a liquid crystal display device using a frontlight can beconstructed in a similar manner as to a structure in which itslight-emitting elements are mounted on its printed circuit board. Inthis case, a light guide is not needed or is disposed on the displayscreen of its liquid crystal display panel, and the printed circuitboard is disposed on the back surface of the liquid crystal displaypanel. In addition, the size of the liquid crystal display panel is notlimited to a small size such as those of mobile telephones or mobileinformation terminals, and the invention can also be applied to a liquidcrystal display panel of comparatively large size.

It goes without saying that the invention is not limited to any of theabove-described constructions nor the construction of any of embodimentswhich will be described later, and various modifications can be madewithout departing from the technical ideas of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily appreciated and understood fromthe following detailed description of preferred embodiments of theinvention when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view diagrammatically showing essentialportions aiding in describing a first embodiment of a liquid crystaldisplay device according to the invention;

FIGS. 2A and 2B are explanatory views of an example of the constructionof the printed circuit board shown in FIG. 1, FIG. 2A being a plan viewseen from the side where the liquid crystal display panel is located,and FIG. 2B being a cross-sectional view taken along line A-A of FIG.2A;

FIG. 3 is a developed perspective view aiding in describing the entireconstruction of the first embodiment of the liquid crystal displaydevice according to the invention;

FIG. 4 is a diagrammatic explanatory cross-sectional view of theessential construction of a second embodiment of the liquid crystaldisplay device according to the invention;

FIGS. 5A to 5C are explanatory views of the advantage of the mountingstructure of LEDs on the printed circuit board which constitutes anauxiliary illuminating device in the liquid crystal display deviceaccording to the invention, FIG. 5A being a schematic plan view showingfor comparison purposes a case in which a printed circuit board does nothave mounting through-holes for LEDs, and FIGS. 5B and 5C beingschematic plan views showing different cases in each of which a printedcircuit board has mounting through-holes for LEDs;

FIGS. 6A to 6C are explanatory views of the shape of the LED to be usedin the auxiliary illuminating device which constitutes the liquidcrystal display device according to the invention, FIG. 6A being a topview, FIG. 6B being a side view, and FIG. 6C being a bottom view;

FIG. 7 is a partial plan view aiding in describing one example of athrough-hole formed in the printed circuit board used in the auxiliaryilluminating device which constitutes the liquid crystal display deviceaccording to the invention;

FIGS. 8A to 8C are explanatory views of another example of the shape ofthe LED to be used in the auxiliary illuminating device whichconstitutes the liquid crystal display device according to theinvention, FIG. 8A being a top view, FIG. 8B being a side view, and FIG.8C being a bottom view;

FIG. 9 is a partial plan view aiding in describing another example ofthe through-hole formed in the printed circuit board used in theauxiliary illuminating device which constitutes the liquid crystaldisplay device according to the invention;

FIGS. 10A and 10B are diagrammatic explanatory cross-sectional views ofthe arrangement relationship between the printed circuit board and LEDs,which arrangement relationship provides another advantage according tothe invention, FIG. 10A being a comparative arrangement viewcorresponding to FIG. 14, and FIG. 10B being a view showing arepresentative arrangement according to the invention;

FIG. 11 is a diagrammatic explanatory perspective view of the entireconstruction of a related-art auxiliary illuminating device in asmall-sized information terminal;

FIGS. 12A and 12B are explanatory views of a structural example in whichLEDs constituting the light source of the related-art auxiliaryilluminating device are disposed in a small-sized information terminal,FIG. 12A being a plan view, and FIG. 12B being a cross-sectional viewtaken along line A-A of FIG. 12A;

FIG. 13 is a diagrammatic explanatory cross-sectional view of an exampleof the essential construction of a proposed example of an auxiliaryilluminating device in a small-sized information terminal;

FIG. 14 is a diagrammatic explanatory view of an example of theessential construction of a liquid crystal display device, aiding inexplaining the problem of the construction shown in FIG. 13; and

FIG. 15 is a diagrammatic explanatory view of an example of theessential construction of another proposed example of the auxiliaryilluminating device in the small-sized information terminal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be described below in detailwith reference to the drawings of the embodiments. FIG. 1 is across-sectional view aiding in describing a first embodiment of a liquidcrystal display device according to the invention, diagrammaticallyshowing the essential parts of the first embodiment. In an auxiliaryilluminating device of the first embodiment, LEDs 8 are disposed aslight-emitting elements at a side edge 4A which is an entrance surfaceof a light guide 4 made of a transparent plate such as an acrylic plate,and light emitted from the LEDs 8 is made to enter and propagate in thelight guide 4. During this propagation, the light is made to exit fromthe obverse side of the light guide 4 (in the direction of a liquidcrystal display panel 1), thereby forming a surface light source whichilluminates the entire surface of the liquid crystal display panel 1from behind the liquid crystal display panel 1.

The LEDs 8 are mounted in the state of being respectively insertedthrough through-holes 7 formed in a printed circuit board 5. Thisprinted circuit board 5 is a flexible printed circuit board. The printedcircuit board 5 of the first embodiment is folded toward the liquidcrystal display panel 1 at an end portion opposite to the side where theLEDs 8 are mounted, and is connected to an electrode terminal led out ofeither one of the substrates of the liquid crystal display panel 1.Incidentally, at an end portion where the LEDs 8 are mounted or an endportion adjacent to the end where the LEDs 8 are mounted, the printedcircuit board 5 may be folded and connected to a terminal of the liquidcrystal display panel 1.

In a light-emitting portion 8A of each of the LEDs 8, in terms of theefficiency of utilization of light, it is desirable that the size of thelight-emitting portion 8A taken in the thickness direction of the lightguide 4 be made not greater than the thickness of the entrance surface4A of the light guide 4. In the first embodiment, both are madeapproximately equal. Other electronic components 9 such as driver ICs,resistors and capacitors are mounted on the printed circuit board 5. TheLEDs 8 are mounted in the same direction as these other electroniccomponents 9, and an electrode 8B provided at the bottom of each of theLEDs 8 is bridged to the mounting surface of the printed circuit board 5and is electrically connected and secured to the printed circuit board5. In other words, such various components (the LEDs 8 and the otherelectronic components 9) are mounted on the printed circuit board 5 bysingle-sided mounting. Incidentally, a diffusing sheet and a prism sheetare interposed between the liquid crystal display panel 1 and theauxiliary illuminating device, but their illustration is omitted in FIG.1.

FIGS. 2A and 2B are explanatory views of an example of the constructionof the printed circuit board shown in FIG. 1. FIG. 2A is a plan viewseen from the side where the liquid crystal display panel is located,and FIG. 2B is a cross-sectional view taken along line A-A of FIG. 2A.Reference numeral 9A is an area in which the other electronic components9 are mounted. It goes without saying that the mounting area 9A is notlimited to the shown portion.

According to the first embodiment, as to the printed circuit board 5 onwhich the light-emitting elements 8 such as LEDs are mounted from thesame side together with the other electronic components 9, thelight-emitting portions 8A of the respective light-emitting elements 8can be disposed in opposition to the entrance surface 4A of the lightguide 4 without the need to fold the printed circuit board 5 in theportion of the light-emitting elements 8, thereby enabling a reductionin the number of process steps. In addition, by using the flexibleprinted circuit board 5, it is possible to easily provide electricalconnection between the liquid crystal display panel 1 and the printedcircuit board 5.

FIG. 3 is a developed perspective view aiding in describing the entireconstruction of the first embodiment of the liquid crystal displaydevice according to the invention. Fixing means such as a spacer ordouble-sided adhesive tape is interposed in the required portion betweeneach constituent element, but their illustration is omitted for ease ofunderstanding. Two prism sheets 2 and a diffusing sheet 3 are insertedat the back of the liquid crystal display panel 1, and the light guide 4is disposed under the diffusing sheet 3. Positioned under the lightguide 4 is the printed circuit board 5 on which the LEDs 8 and the otherelectronic components 9 are mounted.

The liquid crystal display panel 1, the prism sheets 2, the diffusingsheet 3, the light guide 4 and the printed circuit board 5 areaccommodated in a molded case 6 which is a box-like case, to constitutean integral liquid crystal display device. An area shown by dashed linesin the liquid crystal display panel 1 denotes a display area AR, andapproximately coincides with an area to be illuminated by the lightguide 4.

FIG. 4 is a diagrammatic explanatory cross-sectional view of theessential construction of a second embodiment of the liquid crystaldisplay device according to the invention. In FIG. 4, the same referencenumerals as those used in FIG. 1 denote the same functional parts asthose shown in FIG. 1, and although the printed circuit board 5 of thefirst embodiment is a flexible printed circuit board, the secondembodiment uses a hard printed circuit board 50. A connecting flexibleprinted sheet 50A is used to provide connection between the printedcircuit board 50 and the terminal of the liquid crystal display panel 1.

Each of the LEDs 8 has a height which allows for the thickness of theprinted circuit board 50. The LEDs 8 are secured (connected) to theprinted circuit board 50 by the electrodes 8B similar to those used inthe first embodiment. Specifically, LEDs each having a heightcorresponding to the thickness of the printed circuit board 50 are usedas the LEDs 8.

According to the second embodiment, as to the printed circuit board 5 onwhich the LEDs 8 are mounted from the same side together with the otherelectronic components 9, the light-emitting portions 8A of therespective LEDs 8 can be disposed to be accurately directly opposed tothe entrance surface 4A of the light guide 4 without the need to foldthe printed circuit board 5 in the portion of the LEDs 8, therebyenabling a reduction in the number of process steps.

FIGS. 5A to 5C are explanatory views of the advantage of the mountingstructure of the LEDs on the printed circuit board which constitutes theauxiliary illuminating device in the liquid crystal display deviceaccording to the invention. FIG. 5A is a schematic plan view showing forcomparison purposes a case in which a printed circuit board does nothave mounting through-holes for LEDs, while FIGS. 5B and 5C areschematic plan views showing different cases in each of which a printedcircuit board has mounting through-holes for LEDs. Incidentally, each ofthe through-holes 7 shown in FIGS. 5B and 5C is formed to have a sizewhich allows one LED 8 to be inserted through it without substantiallooseness.

As shown in FIG. 5A, in the case where the through-holes 7 are notprovided in the printed circuit board 5, it is difficult to dispose thelight-emitting portions 8A of the LEDs 8 so that the light-emittingportions 8A are accurately directly opposed to the entrance surface 4Aof the light guide 4. Accordingly, the directions in which lightsemitted from the respective LEDs 8 enter the light guide 4 becomeirregular as shown by arrows and the light guide 4 cannot irradiate theliquid crystal display panel 1 with uniform illuminating light, with theresult that display irregularity occurs in the liquid crystal displaypanel 1. In FIG. 5A, sign AR denotes an effective area (effectiveillumination area).

In contrast, as shown in FIG. 5B, in the case where a plurality ofthrough-holes 7 for mounting the LEDs 8 are provided in the printedcircuit board 5 at positions approximately equidistant from an adjacentend of the light guide 4, the distance between each of the LEDs 8 andthe light guide 4 and the mounting attitude of each of the LEDs 8 isrestricted by the corresponding one of the through-holes 7, whereby thedirections in which lights emitted from the respective LEDs 8 enter thelight guide 4 become regular as shown by arrows. In addition, as shownin FIG. 5C, by adjusting distances D1 and D2 of the plurality of LEDs 8to the entrance surface 4A of the light guide 4, namely, by setting thedistances of the respective through-holes 7 to the light guide 4according to a desired luminance distribution of the light guide 4, itis possible to design into a desired pattern the distribution ofilluminating light which exits from the light guide 4 toward the liquidcrystal display panel 1.

FIGS. 6A to 6C are explanatory views of the shape of an LED to be usedin the auxiliary illuminating device which constitutes the liquidcrystal display device according to the invention. FIG. 6A is a topview, FIG. 6B is a side view, and FIG. 6C is a bottom view. This LED 8has convex portions 8C on the (back) side opposite to the light-emittingportions 8A, and also has a pair of electrodes 8B to be connected to thewiring (power supply line) of the printed circuit board 5. Theelectrodes 8B are bridged to the mounting surface of the printed circuitboard 5, and the light-emitting portion 8A is opposed to the entrancesurface 4A of the light guide 4 on the opposite side to the mountingsurface. Incidentally, the direction in which to dispose each of theelectrodes 8B and the shapes thereof are not limited to the shownexample, and may be any direction and any shape that enable theelectrodes 8B to be bridged to the printed circuit board 5 and besecured and electrically connected thereto.

FIG. 7 is a partial plan view aiding in describing one example of thethrough-hole formed in the printed circuit board used in the auxiliaryilluminating device which constitutes the liquid crystal display deviceaccording to the invention. The shown example corresponds to thethrough-hole 7 for mounting the LED 8 described with reference to FIGS.6A to 6C. This through-hole 7 is an approximately rectangular holehaving concave portions 7A which correspond to the respective convexportions 8C of the LED 8 shown in FIG. 6A. The through-hole 7 allows theLED 8 shown in FIGS. 6A to 6C to be inserted through itself, andrestricts the insertion position of the LED 8 to set the distance of theLED 8 to the entrance surface 4A of the light guide 4 to a predeterminedvalue.

FIGS. 8A to 8C are explanatory views of another example of the shape ofan LED to be used in the auxiliary illuminating device which constitutesthe liquid crystal display device according to the invention. FIG. 8A isa top view, FIG. 8B is a side view, and FIG. 8C is a bottom view. ThisLED 8 has concave portions 8D in a direction parallel to the entrancesurface 4A of the light guide 4 (on opposite sides), and also has a pairof electrodes 8B to be connected to the wiring (power supply line) ofthe printed circuit board 5. The electrodes 8B are bridged to themounting surface of the printed circuit board 5, and the light-emittingportion 8A is opposed to the entrance surface 4A of the light guide 4 onthe opposite side to the mounting surface. Incidentally, the directionin which to dispose each of the electrodes 8B and the shapes thereof maybe any arbitrary ones that enable the electrodes 8B to be bridged to theprinted circuit board 5 and be secured and electrically connectedthereto.

FIG. 9 is a partial plan view aiding in describing another example ofthe through-hole formed in the printed circuit board used in theauxiliary illuminating device which constitutes the liquid crystaldisplay device according to the invention. The shown example correspondsto the through-hole 7 for mounting the LED 8 described with reference toFIGS. 8A to 8C. This through-hole 7 is an approximately rectangular holehaving convex portions 7B which correspond to the respective concaveportions 8D of the LED 8 shown in FIG. 8A. The through-hole 7 allows theLED 8 shown in FIGS. 8A to 8C to be inserted through itself, andrestricts the insertion position of the LED 8 to set the distance of theLED 8 to the entrance surface 4A of the light guide 4 to a predeterminedvalue.

Since each of the LEDs 8 and the corresponding one of the through-holes7 of the printed circuit board 5 are shaped to have positioning meanssuch as that shown in FIGS. 6A to 6C and 7 or that shown in FIGS. 8A to8C and 9, the distance between each of the LEDs 8 and the entrancesurface 4A of the light guide 4 and the opposition attitude of each ofthe LEDs 8 can easily be accurately set to predetermined values,respectively, whereby the distribution of light entering the light guide4 is prevented from being disturbed. Accordingly, the liquid crystaldisplay panel 1 can be illuminated with light of predetermineddistribution, whereby a high-quality image display can be obtained.

FIGS. 10A and 10B are diagrammatic explanatory cross-sectional views ofthe arrangement relationship between the printed circuit board and theLEDs, which arrangement relationship provides another advantageaccording to the invention. FIG. 10A is a comparative arrangement viewcorresponding to FIG. 14 referred to previously, and FIG. 10B is a viewshowing a representative arrangement according to the invention. Thenumerical values shown in FIGS. 10A and 10B are mere examples, and theirunits are millimeters.

In this example, the thickness of the printed circuit board 5 or 50 is0.2 mm, the entire height of each of the LEDs 8 is 1.0 mm, theheightwise size of the light-emitting portion 8A of each of the LEDs 8is 0.6 mm, and the size between the top end of the LED mold and that ofthe light-emitting portion 8A and the size between the bottom end of theLED mold and that of the light-emitting portion 8A are 0.2 mm,respectively. As shown in FIG. 10A, in the case where the LEDs 8 aremounted on the surface of the printed circuit board 5 (50), thethickness of the light guide 4 is limited to 1.0 mm or less when thebalance between the efficiency of utilization of emitted light and lightentering the light guide 4 is taken into account in the thicknessdirection (in the vertical direction as viewed in FIG. 10A).Incidentally, if such balance is not taken into account, the thicknessof the light guide 4 is reduced to 0.8 mm by 0.2 mm at the top end ofthe LED mold.

In contrast, according to the invention shown in FIG. 10B, the portionof the LED mold which does not contribute to light emission is embeddedinto the printed circuit board 5 (60) so that the position of the bottomend (lying on the side of the printed circuit board 5 (50)) of thelight-emitting portion 8A of each of the LEDs 8 can be made close to thesurface of the printed circuit board 5 (50). Accordingly, if theposition of the bottom end is made approximately coincident with thesurface of the printed circuit board 5 (50), the thickness of the lightguide 4 can be reduced to 0.6 mm. Therefore, in this case, the totalthickness of each of the LEDs 8 and the light guide 4 can be reduced to0.8 mm, whereby the thickness of the entire liquid crystal displaydevice including the auxiliary illuminating device can be made small.According to each of the above-described embodiments of the invention,it is possible to provide a liquid crystal display device capable ofproviding image display with high luminance free of display irregularityor with good color balance. Incidentally, it goes without saying thateven if the position of the bottom end of the light-emitting portion 8Ais not made coincident with the surface of the printed circuit board 5(50), the advantage of a reduction in thickness can be obtained bymounting the respective LEDs 8 in the through-holes 7.

As is apparent from the foregoing description, according to theinvention, the light-emitting elements 8 of the auxiliary illuminatingdevice and the entrance surface (side edge) 4A of the light guide 4 caneasily be accurately directly opposed to one another, whereby theluminance irregularity of illuminating light can be reduced and theefficient utilization of light exiting from the light-emitting elements8 can be promoted.

In addition, the highly efficient utilization of light entering thelight guide 4 is enabled, whereby luminance can be improved and thedisplay irregularity of the liquid crystal display panel 1 can bereduced.

1. A liquid crystal display device comprising: a liquid crystal displaypanel; a printed circuit board; a light guide being disposed at a backside of the liquid crystal display panel; and a light-emitting element,wherein the printed circuit board includes a through hole at a portionapproximately parallel with the liquid crystal display panel, theprinted circuit board is a flexible printed circuit board and iselectrically connected to the liquid crystal display panel directly, thelight-emitting element includes a mold, a light-emitting portion andelectrodes, the electrodes of the light-emitting element are connectedto the printed circuit board, at least a portion of the mold of thelight-emitting element is disposed in the through-hole, and thelight-emitting portion is disposed out of the through-hole to face aside edge of the light guide and to project a light into the light guidein a direction approximately parallel with the printed circuit board. 2.A liquid crystal display device according to claim 1, wherein theelectrodes of the light-emitting element are connected to the printedcircuit board at a first side of the printed circuit board, and thelight-emitting portion of the light-emitting element is disposed at asecond side of the printed circuit board opposed to the first side.
 3. Aliquid crystal display device according to claim 2, further comprisingan electronic component other than the light-emitting element, whereinthe electronic component is mounted on the first side of the printedcircuit board.
 4. A liquid crystal display device according to claim 3,the light guide is disposed between the liquid crystal display panel andthe printed circuit board.
 5. A liquid crystal display device accordingto claim 1, the light guide is disposed between the liquid crystaldisplay panel and the printed circuit board.
 6. A liquid crystal displaydevice comprising: a liquid crystal display panel; a printed circuitboard; a light guide being disposed at a back side of the liquid crystaldisplay panel; and a light-emitting element, wherein the printed circuitboard includes a through hole at a portion approximately parallel withthe liquid crystal display panel, the printed circuit board iselectrically connected to the liquid crystal display panel via aconnecting flexible printed circuit board, the light-emitting elementincludes a mold, a light-emitting portion and electrodes, the electrodesof the light-emitting element are connected to the printed circuitboard, at least a portion of the mold of the light-emitting element isdisposed in the through-hole, and the light-emitting portion is disposedout of the through-hole to face a side edge of the light guide and toproject a light into the light guide in a direction approximatelyparallel with the printed circuit board.
 7. A liquid crystal displaydevice according to claim 6, wherein the electrodes of thelight-emitting element are connected to the printed circuit board at afirst side of the printed circuit board, and the light-emitting portionof the light-emitting element is disposed at a second side of theprinted circuit board opposed to the first side.
 8. A liquid crystaldisplay device according to claim 7, further comprising an electroniccomponent other than the light-emitting element, wherein the electroniccomponent is mounted on the first side of the printed circuit board. 9.A liquid crystal display device according to claim 8, the light guide isdisposed between the liquid crystal display panel and the printedcircuit board.
 10. A liquid crystal display device according to claim 6,the light guide is disposed between the liquid crystal display panel andthe printed circuit board.
 11. A liquid crystal display device accordingto claim 6, the printed circuit board is a hard printed circuit board.